The present invention relates to a semiconductor laser device and, more particularly, an embedded type of semiconductor laser device and a process for making such a device.
FIG. 2 illustrates a typical structure of a conventional semiconductor laser device in which the transverse mode is controlled by the provision of an embedded layer. In FIG. 2, reference numeral 22 stands for a semi-insulating substrate, 23 a lower clad layer, 24 an active layer, 25 an upper clad layer, 26 a p-type carrier injection layer, 27 a blocking layer, 27 an n-type carrier injection layer, and 29 and 30 electrodes.
The laser device shown in FIG. 2 is of the structure that carriers are transversely injected in the active layer. The lower clad layer 23 of Al.sub.x Ga.sub.1-x As, the active layer 24 of Al.sub.y Ga.sub.1-y As and the upper clad layer 25 of Al.sub.x Ga.sub.1-x As, provided that x&gt;y, are laminated on the semi-insulating substrate 22 of GaAs in that order. The active layer 24 may be not only of the above structure but also of a singlet or multiplet quantum well structure or a multiplet active layer structure thereof.
FIG. 3 is a view for illustrating a conventional multiplet quantum well structure.
When the thickness of the active layer is 20 nm or below, electrons occupy only limited levels due to the resulting quantum effect. For that reason, transition occurs on the thus limited levels so that the efficiency of radiation is increased, resulting in a decrease in threshold value currents. The multiplet quantum well structure makes use of such phenomena. As illustrated, a very small thickness is given to an active layer 33 formed between upper and lower clad layers 31 and 32, while barrier layers 34 of AlGaAs and quantum wells 35 of GaAs are repeatedly formed. To effectively confine light in the active layer, the mixed crystal ratio of Al of the barrier layers is smaller than that of the clad layers. Usually, certain limitations are imposed upon the number of wells, the thickness of barrier layers and the mixed crystal ratio of barrier layers (generally, the number of wells is 5, the thickness of barrier layers is 6 to 20 nm and the Al to As ratio of barrier layers is 0.2 to 0.3). This is because in the case of the structure where currents flow in the longitudinal direction, there is a change in the injection of carriers in the upper and lower quantum wells. However, the transverse junction type laser device is freed of such limitations. Thus, an energy band structure of a conduction band Ec and a valence band Ev is obtained, as illustrated. Carriers are injected in the quantum wells, so that radiation takes place due to the recombination of electrons with holes, and the coherency of output light is maintained by reason that the quantum wells interact. It is noted that the reason why the space between the clad layer and the first quantum well is increased is to prevent the similarity of other quantum wells to the energy band structure from deteriorating because of the energy level of the clad layer being high. By using the quantum wells for the active layer, it is also possible to obtain a very decreased inter-electrode capacity.
Usually, regions for embedding p- and n-type carrier injection layers 26 and 28 are formed by the LPE (liquid-phase epitaxial) technique. A blocking layer 27 is provided to prevent any leak from occurring between the p- and n-type carrier injection layers 2 through a substrate.
However, difficulty is involved in the formation of the regions for embedding carrier injection layers by the LPE technique, since a melt tends to be oxidized to form a surface oxide film on Al.sub.x Ga.sub.1-x As, if the value of x is increased as is the case with the clad layers of semiconductor laser devices. For that reason, it has been required to previously mesa-etch a region to a depth reaching the GaAs substrate 22 and embed a carrier layer in the GaAs substrate 22.
When currents flow in the transverse direction in this structure, leak currents occur through the substrate. In order to prevent this, it is required that the blocking layer 27 be provided in either one of the embedding regions (in FIG. 2, a p-type AlGaAs layer having a molar Al to As ratio of 0.2 to 0.4 be provided in the lower portion of the n-side carrier injection layer 27). It is difficult to form a blocking layer having a uniformly controlled large area, as experienced with a 2- or 3-inch substrate, thus leading to a lowering of device process yields.